Under current production methods the low freeness (drainage) of secondary fiber pulp such as deinked pulp necessitates the use of polymer drainage aids and/or mixing of the deinked pulp with virgin softwood in order to run the pulp on conventional paper making equipment. The inclusion of a high percentage of softwood in the blend reduces the commercial value of the pulp since it can not be classified as "deinked". Further drainage-aid polymers used to enhance freeness of the pulp trap residual ink particles causing a loss of brightness, again lowering of the commercial value of the resulting pulp.
Use of enzymes for freeness enhancement is also known in the prior art, in particular, in the paper making industry it is known that cellulases alone or in conjunction with hemicellulases (which class of enzymes include xylanase) enhance the freeness of the treated pulp. U.S. Pat. Nos. 4,923,565, 5,110,412, 5,308,449 to Fuentes et al. and French patent 2,604,198 are representative of methods which disclose treating recycled fibers in aqueous suspension with cellulases and/or hemicellulases to improve drainage of the pulps to improve the mechanical characteristics of the paper.
U.S. Pat. Nos. 5,169,497, 5,423,946, and 5,501,770 to Sarkar all disclose the use of a cellulolytic enzyme in conjunction with an acrylamide copolymer to improve freeness (drainage). The Sarkar disclosures require the presence of both the enzyme and polymer for significant freeness (drainage) enhancement of pulp prepared from a mixture of old corrugated boxes and old newsprint.
U.S. Pat. No. 5,116,474 to Fuentes and U.S. Pat. No. 5,364,501 to Baret both disclose the use of alkaline cellulases to improve drainage on the deinked wastepaper, however, both patents require treatment at an alkaline pH with a specific alkaline cellulase.
An article by Pommier, Paper Technology, p.50, October 1991, showed use of a cellulase product called Liftase A40 (now called Pergalase.RTM. A40 available from Ciba-Geigy, Greensboro, N.C., 27419) for use in the pulp and paper industry in deinking and refining processes.
In general the prior art has shown that a gain of &gt;50 csf(Canadian Standard Freeness) points is obtainable by treatment with cellulase. However, comparative experiments conducted indicated that use of cellulase alone also lead to a loss in pulp brightness. Further when used in combination with a drainage aid polymer the loss in brightness was even more pronounced. The mixture of enzymes provided by the invention accomplish the goal of simultaneously increasing freeness (drainage) without loss of brightness and physical properties.
Other known enzyme treatments to enhance freeness include the use of pectinases. U.S. Pat. No. 5,487,812 to Thornton discloses adding pectinase to the aqueous phase of bleached or alkaline treated pulp to improve drainage. The discussion of related technology in Thornton refers to a published Japanese patent application 2,118,191 which notes the improvement in brightness of pulp treated with pectinase. However, the disclosure in the Thornton '812 patent explains that the stage in the process at which the pectinase is to be added would not preclude subsequent release of pectins which would interfere with brightness.
The article by J. Thornton in TAPPI Journal, Vol. 77, No. 3 (March 1994) discloses pectinase treatment of bleached pulp suspensions to dissolve polygalacturonic acids to prevent them from complexing with cationic polymers.
The use of a combination of enzymes are disclosed in U.S. Pat. No. 4,891,096 to Akkawi and U.S. Pat. No. 5,103,883 to Viikari which describe use of pectinase, hemicellulase and cellulase enzymes in combination for biochemical retting of hemp and for debarking of logs, respectively.
Unlike the known prior art processes, the present invention improves the drainage ("freeness") of pulp made from secondary fiber, such as deinked pulp (DIP) by treatment with a combination of cellulase and pectinase enzymes. Such treatment is advantageous over known processes by providing a process where there is no loss in pulp strength properties or brightness and the enzymatically treated secondary fiber pulp can be run on conventional paper making equipment without the addition of softwood pulp. In some instances, for strength properties, addition of softwood pulp may be desired. However, the invention process provides that a lesser amount of softwood pulp is needed than required by presently known processes.
Accordingly, it is a broad object of the invention to provide a process to increase the freeness (drainage) of pulp made from secondary fiber while retaining other important characteristics such as pulp brightness and strength. Pulp brightness is not retained when using presently known drainage chemicals. These are important factors which impact the utility of the pulp in conventional paper making operations and marketability.
Another object of the invention is to produce a pulp having enhanced freeness and high brightness made from secondary fiber which has been enzymatically treated.
A specific object of the invention is to provide a process for enhancing the freeness of secondary fiber pulp without the presence of polymer drainage aids and/or the addition of softwood pulp.
Another specific object of the invention is to provide a process which provides a high pulp yield after enzymatic treatment.